Recently, as electronic appliances are becoming smaller in size and thickness, and higher in function, a high wiring accommodation capacity is demanded in a circuit board for composing an electronic circuit of an electronic appliance. In particular, to meet the need for higher mounting density, a multi-layer printed wiring board called mother board is further combined with a circuit board mounting semiconductors and other components.
In mounting of electronic components relating to small electronic devices such as portable telephone or digital still camera, or RF or other various modules and LED, in order to reduce the height of a circuit board after mounting electronic components, a multi-layer circuit board having a cavity structure is much expected, and various boards are developed, such as low temperature co-fired ceramic (LTCC) board, solid circuit board by resin forming, and multi-layer circuit board having cavity structure or component incorporating structure.
FIG. 10A is a sectional view of a conventional ceramic circuit board. The sectional structure of a conventional LTCC or other ceramic board is a multi-layer circuit board as shown in FIG. 10A, and green sheet 50 forming wiring conductors, or blanked holes or cavities as openings 55 in a ceramic board is laminated in a plurality, and fired and formed. In this case, usually, the low temperature co-fired ceramic board is fired at 900° C. or less, and the glass ceramic board is fired at 1000° C. or less.
FIG. 10B is a sectional view of a conventional resin-made circuit board. The conventional solid multi-layer circuit board by resin forming is as shown in FIG. 10B, and resin forming is generally achieved by forming mold resin layer 51 on lower substrate 52, heating and fusing the resin in a die or the like, and forming a wiring circuit on the surface by plating.
However, this ceramic board must be fired at high temperature near 900° C. even in the case of low temperature co-fired ceramics, and due to effects of shrinkage of green sheets, it is hard to assure the dimensional precision and the precision of the circuit. Further, including the formation of cavity structure, the manufacturing lead time is long, and the manufacturing cost is relatively high.
In the case of a multi-layer circuit board forming a cavity structure by resin forming, if through-holes are formed before forming, the through-holes may be deformed by flow of the resin when forming, and deterioration of insulation or short-circuiting between circuits may occur. Accordingly, in the technology of interlayer connection of forming all layers in inner via hole (IVH) structure, there were problems in process or structure. To solve the problems, after resin forming, nonthrough-holes or through-holes may be formed, and through-holes may be formed by conductive plating or by using conductive substance, but it is hard to process in the case of small holes, and it is hard to realize a very fine specification demanded recently.
Besides, the expansion coefficient of the ceramic board or circuit board by resin forming, and the expansion coefficient of mother board (multi-layer printed wiring board) for mounting a board are largely different in the value, there were various restrictions for mounting these boards on the mother board.
Hitherto, a plurality of circuit boards substantially made of same material as multi-layer printed wiring board as the mother board were laminated by way of an adhesion layer such as prepreg sheet, and a multi-layer circuit board having a cavity structure was developed. The prior art documents relating to the invention of the present application are disclosed in, for example, patent document 1 and patent document 2.
However, in the conventional multi-layer circuit board using the prepreg sheet, although the adhesion strength between circuit boards can be assured, the resin may flow out from the adhesion layer of prepreg sheet into the cavity when heating and pressing them, and defects may occur when mounting components in the cavities, and same as the circuit board by resin forming above, since the resin flow occurs, it was extremely difficult to connect all layers in IVH connection, both in structure and manufacturing process.    Patent document 1: Unexamined Japanese Patent Publication No. H9-199824    Patent document 2: Unexamined Japanese Patent Publication No. 2007-59844